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Notes for the Season: July-September 2017

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Notes for the Season features information on meteor showers, space exploration events, and astronomy-related anniversaries from July through September 2017.

Sunrise/Sunset

Sunrise Solar NoonSunset

July 1 5:52 am PDT 1:13 pm PDT 8:36 pm PDT

August 1 6:14 am PDT 1:16 pm PDT 8:18 pm PDT

September 1 6:40am PDT 1:09 am PDT 7:38 pm PDT

(Times are for San Francisco, CA, and will vary slightly for other locations.)

Notes for Summer

On what corresponds to July 4, 1054 AD, Chinese astronomers recorded the appearance of a "guest star" said to have occurred during the 32nd year of the reign of the Song Dynasty's fourth Emperor. This "guest star" was reportedly visible in broad daylight for about three weeks, and after it faded remained bright enough to be seen in the night sky for about two years.

Astronomers studying the location of the "star" in the modern constellation of Taurus the Bull now observe the remnant of a supernova—the catastrophic explosion of a massive, aging star. Known as M-1 in Charles Messier's catalog of non-stellar objects, it was named the "Crab Nebula" by William Parsons in 1840 and can be observed in modest telescopes as a tiny, diffuse spot one degree from the star Zeta Tauri, the tip of the southern horn of Taurus. In fact, the roughly-oblong nebula measures about 8-by-11 light years across. It's so far away—39 million billion miles—that its light takes about 6,500 years to reach us, so astronomers say that it's 6,500 located light years away.

In 1967, Jocelyn Bell Burnell discovered a rapidly-pulsating radio source at the heart of the Crab Nebula, now known to be a neutron star with about 1.5 times the mass of the Sun compressed to a 16 kilometer (10 mile) diameter, spinning 30 times per second.

July 20 is the 48th anniversary of the first manned landing on the Moon, which took place in 1969. Astronauts Neil Armstrong and Edwin Aldrin, aboard the Apollo 11 lunar module "Eagle," set down gently on the Moon's Sea of Tranquility, while the third member of the Apollo crew, command module pilot Michael Collins, remained in orbit aboard the mothership "Columbia." Armstrong and Aldrin stayed on the surface for nearly a full day, 2.5 hours of which were spent outside the spacecraft, where they conducted experiments, took photos, and collected about 40 pounds of surface material. The landing site has since been photographed from lunar orbit by the Lunar Reconnaissance Orbiter.

August 7's partial lunar eclipse is centered over the Indian Ocean when it takes place (and therefore when the Moon is still below the horizon for skywatchers in the US). The full Moon passes partly through the dark central portion of Earth's shadow (the umbra), but never enough to be completely immersed. At maximum, the shadow of our planet will intrude from the south polar region, extending only about a quarter of the way across the Moon's disk. For those able to view the eclipse, this is a good opportunity to observe the curvature of Earth's shadow. This is seen during every umbral lunar eclipse and is proof that Earth is a sphere, since the only object that always casts a round shadow, regardless of what direction light is shining from, is a sphere.

August 12 is the peak date of the Perseid meteor shower, caused when Earth passes through the dust trail of Comet 109P/Swift-Tuttle, causing them to slam into the atmosphere at nearly 40 miles per second. This heats them to incandescence, causing the streaks of light known as "meteors." The Perseids are active from about July 13 to August 26, slowly building to a peak on August 12, when the display averages 60-80 meteors per hour under ideal conditions (in other words, as seen from a rural observing site with no Moon in the sky). Unfortunately, this year's Perseid-viewing prospects are less than ideal: the waning gibbous Moon rises just before midnight and is high in the sky at dawn. This means that moonlight will wash some meteors from view, reducing the observable rate to perhaps 10-15 per hour. However, keep in mind that prudent meteor-watchers bracket the peak date with additional observing nights before and after, just in case.

Have any plans for the morning of August 21? Many will be observing the "Great American Solar Eclipse," as the new Moon passes between our planet and its star, casting its shadow onto Earth's surface and causing a total solar eclipse. Not since 1979 has the path of a total eclipse touched American soil, and that time, it curved only over the northwest, passing over Washington, Oregon, Idaho, Montana, and North Dakota. This time, the shadow makes a long, diagonal slash across the country, starting in Oregon and moving into Idaho, crossing Wyoming, Nebraska and Missouri to parts of Kentucky, Tennessee, northern Georgia, and finally leaving the East Coast in South Carolina. As seen from along that narrow path, the Moon will completely block the Sun from view, allowing its softly-glowing outer atmosphere, or "corona," to be seen.

Observers in areas not located precisely on the path of totality will experience a partial eclipse, depending on how far they are from the shadow's path. As seen from San Francisco, the Moon will encroach across 80% of the Sun's disk, making the Sun look like a crescent. This crescent will still be bright enough to wash the corona from view, and Morrison Planetarium staff will be hosting safe observations with the public from the Academy and showing a live-stream of the eclipse from the Internet in case local skies are cloudy.

During the predawn hours of September 12, observers in the western US and Hawai'i will see the waning gibbous Moon move in front of the star Aldebaran (the eye of Taurus the Bull), hiding it from view for 79 minutes. This event is known as an "occultation," and Aldebaran is one of only a few bright, easily-visible stars close enough to the Moon's path across the sky that can be occulted. As seen from San Francisco, it begins at 4:29 am and ends at 5:48 am. Timings for other cities can be found here. Occultations can reveal the presence (or absence) of an atmosphere, depending on how long it takes the star to disappear from view. Aldebaran winks out quickly as the Moon moves in front of it. What does that tell you about the Moon's atmosphere?

The autumnal equinox for the Northern Hemisphere arrives on September 22 at 1:02 pm PDT. That's when Earth is at the point in its orbit where neither pole is pointed more toward the Sun than the other. From Earth's perspective, the Sun crosses the celestial equator from the northern half of the sky to the southern half, and day by day, the period during which the Sun is above the horizon is gradually shortening. This is often said to be when day and night are of equal length, but that's only in theory, where the Sun is considered to be a single point and optical effects caused by Earth's atmosphere aren't taken into account. In reality, the combination of our star's apparent diameter in the sky and refraction by the atmosphere cause some portion of the Sun to be visible above the horizon for a few minutes longer than it is hidden below. For the northern mid-latitudes, day and night are closest to being equal in length on about the 26th, a couple of days after the fall equinox.

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